We are continuing to develop a digital feedback tracking system which, in conjunction with our second generation optical trapping rig, will allow us to study the diffusion and directed motion of cell membrane proteins. With this system we will be able to distinguish random diffusion from directed motion, investigate the nature of anomalous diffusion on cell membranes, and gain information on the viscous drag of the membrane and glycocalyx on cell surface proteins. Initial experiments will take place using IgE proteins on RBL cells; later investigations may involve LDL-receptor molecules, NCAMs, or MHC class 1 molecules.